1. Field of the Invention
The present invention relates to a multilayer substrate for use in a satellite broadcast reception apparatus, or a low noise block-down (LNB) converter, employed in satellite broadcasting and satellite communication, and a satellite broadcast reception apparatus (hereinafter, referred to as the xe2x80x9cLNB converterxe2x80x9d) using the relevant multilayer substrate.
2. Description of the Background Art
As a substrate for use in the LNB converter, a so-called double-layer substrate (double-sided substrate) has been employed conventionally. As shown in FIG. 10, the double-layer substrate has a substrate dielectric layer 105 formed of a base material of teflon (R) having its both sides plated with copper to form microstrip lines 101, 102. In the microstrip lines, as shown in FIG. 11, a circuit pattern to be a signal line is formed on the front layer of the substrate, and the entire rear layer is grounded, to obtain low-loss and stable transmission characteristic. That is, as shown in FIG. 12, a radiowave signal having propagated through a waveguide 113 is introduced into a space surrounded by a frame 112 and a chassis 111, and is transmitted via a probe 120 into the microstrip line formed in the front layer 101 of double-layer substrate 110. FIG. 13 shows the structure of a portion of the double-layer substrate in FIG. 12 in the vicinity of the transmitting portion. A projecting portion 101a constituting probe 120 projecting into a through hole 121 is provided on a projecting portion of substrate dielectric layer 105 continued from the signal line of the microstrip line. In the case where a circuit of the LNB converter is formed on the substrate, it is still advantageous to employ the double-layer substrate in that the entire rear side of the substrate is grounded to keep the overall earth state constant.
With recent advancement of multi-channel satellite broadcasting and satellite communication as well as reception from multiple satellites, there is a trend to replace a reception system using a plurality of LNB converters with a reception system using a signal LNB converter. Such an LNB converter in the future will be required to accommodate conventional LNB circuits within a single LNB converter without any problems. Further, the future LNB converter will have to switch and distribute signals for output. Thus, good isolation of the signals preventing interference therebetween will be highly required.
With the conventional double-layer substrate used in the LNB converter, signal lines inevitably cross with each other. Thus, a method to forcibly separate signals using a semi-rigid cable or the like has conventionally been employed. In the LNB converter adapted to the recent multi-satellite system, the signal crossing portion has become further complicated, of which assembly is physically difficult.
A possible solution therefor is to use a multilayer substrate as shown in FIG. 14. This multilayer substrate has pattern layers of first layer 101, second layer 102, third layer 103 and fourth layer 104, with two substrate dielectric layers 105, 107 and a bonding insulating layer 106 arranged therebetween. The multilayer substrate, as shown in FIG. 15, has two double-layer substrates as its base, which are bonded together by the bonding insulating layer 106. Thus, this multilayer substrate has a four-layer configuration, with a front pattern and a rear pattern included in the respective double-layer substrate. It is also possible to provide multilayer configurations of four layers, six layers and eight layers, by laminating the corresponding numbers of double-layer substrates.
In the multilayer substrate for use in the LNB converter, parts can be mounted on the outermost layers or the surface layers, e.g., the first and fourth layers in the case of the four-layer substrate shown in FIG. 16. The microstrip line pattern can also be formed in the surface layers. At this time, a pattern corresponding to a ground layer with respect to the microstrip line is formed in an inner layer that is unseen from the surface of the substrate, e.g., in the second and third layers in the case of the four-layer substrate.
With the multilayer substrate formed by laminating double-layer substrates as described above, however, the ground layer arranged in the inner layer is electrically isolated from an enclosure to which the substrate is secured. Thus, it is likely to suffer transmission loss especially with a high frequency, which becomes the stumbling block preventing the use of the multilayer substrate compared to the double-layer substrate.
As described above, the connection between the probe and the substrate circuit in the double-layer substrate as in FIGS. 12 and 13 permits low-loss power supply. This is because the portion in the vicinity of the probe is surrounded by the metal chassis and frame to prevent radiowave leakage and thus to minimize the transmission loss. With the multilayer substrate, e.g., the four-layer substrate, if the microstrip line pattern is provided in the first layer and the ground pattern is provided in the second, inner layer, the configuration as in FIGS. 12 and 13 cannot be provided. In this case, intervention of the third and fourth pattern layers results in considerable degradation of transmission characteristic. That is, in the case of the double-layer substrate, the double-layer substrate is sandwiched between the chassis and the frame to prevent the radiowave leakage, and, at the same time, the ground for the probe and the ground surface of the circuit connecting portion are commonly provided to sufficiently stabilize the earth. By comparison, in the case of the four-layer substrate, it is difficult to ensure good contact between the ground for the circuit provided in the inner pattern layer and the ground for the chassis. That is, an additional double-layer substrate is inserted between the ground pattern on the rear surface of the double-layer substrate and the chassis, considerably degrading the transmission characteristic. Currently, this obstructs the use of the multilayer substrate in the LNB converter.
An object of the present invention is to provide a multilayer substrate which prevents noise and crosstalk that are likely to occur in multichannel transmission/reception and which ensures transmission loss as low as when one double-layer substrate is employed, and an LNB converter using such a multilayer substrate.
The LNB converter according to the present invention is provided with a multilayer substrate having a microstrip line and a probe, and causes a radiowave signal from an antenna to propagate through a waveguide to transmit through the probe to the microstrip line. The multilayer substrate is provided with a wave-guiding through hole, and the probe is provided to project from the multilayer substrate into the through hole. Of the multilayer substrate, a pattern layer constituting a surface layer on one side is provided with a signal line of the microstrip line and a projecting portion constituting a portion of the probe. Another pattern layer provided with a ground pattern corresponding to the signal line has a portion at least overlapping a root portion of the projecting portion as seen from the top and constituting a surface layer on the other side opposite to the root portion. There is no other pattern layer interposed between the root portion and the portion constituting the surface layer on the other side.
The portion constituting the surface layer on the other side opposite to the root portion may be a portion of a substrate surface layer (the undermost layer) on the other side with respect to the substrate surface layer (the topmost layer) of the multilayer substrate in which the projecting portion of the probe is provided. Alternatively, it may be a portion of an inner pattern layer that is exposed by removing a corresponding portion of the undermost layer to provide a surface layer.
In other words, the present invention provides any of the following structures (1) through (3) in the LNB converter configured with the multilayer substrate, to reduce transmission loss of a high-frequency signal. (1) The structure in which a ground layer is formed of a substrate surface pattern that can contact the earth of the enclosure such as a chassis, rather than an inner pattern layer. (2) The structure in which a ground layer corresponding to the signal line is formed by processing and exposing the inner pattern layer, which is made to contact the enclosure such as a chassis. (3) The structure in which the multilayer substrate as in the structure (1) is employed to form the ground portion with the substrate surface layer pattern, to reduce transmission loss and to facilitate assembly of the LNB converter.
With the above-described structures (1) through (3), the advantageous features of the multilayer substrate can be enjoyed to the full extent, while ensuring low-loss transmission characteristic. That is, the LNB converter employing the multilayer substrate has the advantages including downsizing by virtue of the multi-layered structure, simplification of complicated wiring, facilitation of assembly, and improvement of reliability. This can generally lead to reduction of manufacturing cost, although the substrate cost is currently still expensive as a single item, with only a small number of such substrates available.
The present invention provides the following effects. In an LNB converter receiving signals from multiple satellites where the signals inevitably cross with each other, the external cable conventionally employed becomes unnecessary. Instead, the multilayer substrate, simple in assembly, can be employed to realize an LNB converter ensuring high-level isolation and high performance with less transmission loss. As an increasing number of multilayer substrates are adapted to the LNB converters, the substrate unit price will decrease, which will further promote downsizing of the existing LNB converters.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.